Porcine reproductive and respiratory syndrome virus (PRRSV) is an important pathogen that endangers the swine industry worldwide. Recently, lineage 1 PRRSVs, especially NADC30-like PRRSV, have become the major endemic strains in many pig-breeding countries. Since 2016, NADC30-like PRRSV has become the predominant strain in China. Unfortunately, current commercial vaccines cannot provide sufficient protection against this strain. Here, an attenuated lineage 1 PRRSV strain, named SD-R, was obtained by passaging NADC30-like PRRSV strain SD in Marc-145 cells for 125 passages. Four-week-old PRRSV-free piglets were vaccinated intramuscularly with 10 5.0TCID 50 SD-R and then challenged intramuscularly (2 ml) and intranasally (2 ml) with homologous NADC30-like PRRSV SD (1×10 5.0TCID 50/ml) and heterologous NADC30-like PRRSV HLJWK108-1711 (1×10 5.0TCID 50/ml). The results showed that 5 of 5 immunized piglets had a perceptible humoral immune response to vaccination and did not develop fever or clinical diseases after NADC30-like PRRSV challenge. Additionally, compared to challenge control piglets, vaccinated piglets gained significantly more weight and showed much milder pathological lesions. Furthermore, the viral replication levels of the immunized group were significantly lower than those of the challenge control group. These results demonstrate, for the first time, that lineage 1 PRRSV SD-R is a good candidate for an efficacious vaccine against NADC30-like PRRSVs.

NADC34-like PRRSV strains were first detected in China in 2017, with epidemic potential. In this study, the phylogenetic, epidemic, and recombinant properties of NADC34-like PRRSV in China were evaluated comprehensively. From 2020 to October 2021, 82 NADC34-like PRRSV isolates were obtained from 433 PRRSV-positive clinical samples. These strains accounted for 11.5% and 28.6% of positives in 2020 and 2021, respectively, and have spread to eight provinces. We selected 15 samples for whole-genome sequencing, revealing genome lengths of 15,009 to 15,113 nt. Phylogenetic analysis revealed that Chinese NADC34-like strains cluster with American sublineage 1.5 strains and do not form an independent branch. Recombination analysis revealed that six of fifteen complete genome sequences derived from recombination between NADC34-like and NADC30-like or HP-PRRSV; they all recombined with local strains in China, exhibiting a complex recombination pattern. Partial Nsp2 sequence alignment showed that nine of fifteen isolates have a continuous 100-aa deletion (similar to IA/2014/NADC34); other isolates have a 131-aa discontinuity deletion (similar to NADC30). Five of them also have additional amino acid deletions, all of which are reported for the first time here. In the last two years, NADC34-like PRRSV has become one of the main epidemic strains in some areas of China; it has changed significantly, its homology has decreased significantly, and it has undergone complex recombination with local Chinese strains. These results are of great significance for understanding the current epidemic situation of PRRSV in China.